1. Field of the Invention
This invention relates to optical fibre feedthroughs.
2. Related Art
A device package that incorporates an optical or opto-electronic component has an aperture for a feedthrough feeding one or more optical fibres that conduct light to or from inside of the package. One approach is to seal the optical fibres hermetically within a metallic sleeve with glass solder, which sleeve is, in turn, mounted in the aperture.
Generally `single mode` optical fibre supports two orthogonally polarized He.sub.11 modes. Because of the near-degeneracy of the propagation constants B.sub.X and B.sub.y, for the x and y axes respectively, any small perturbations along the length of the fibre lifts this degeneracy and thus the state of polarization (SOP) will evolve unpredictably. Any linearly polarized light injected into the fibre will thus become elliptically polarized after a short distance.
Many applications, e.g. lithium niobate components and coherent optical transmission systems, require a stable SOP output from the fibre. One solution is to use High Birefringence Polarization Maintaining (HB-PM) fibre.
PM fibres maintain the state of polarization by introducing a large intrinsic birefringence arising from asymmetry in the core or stress effects due to the fibre cladding. The latter of the two methods includes, for example, the formation of high stress regions by means of circular rods along the length of the fibre either side of the core. When launched into one or other of the principal modes, light will propagate unchanged along the length of the fibre provided no external perturbations exceed the internal intrinsic stresses, otherwise the SOP will be unpredictable. When using circular profile tubes it is imperative that the fibre is concentric within the tube, that the tube wall thickness is uniform and that no air bubbles or contaminants interfere with the glass seal. Any significant deviation from a symmetrical structure, both longitudinally and axially, will destroy the symmetry of the stresses external to the fibre and could therefore act to destroy the fibres PM properties. Keeping the symmetry accurate to such an extent can pose difficult problems if large-quantity production is envisaged.